The heat was on once again here in the quarter-finals and although the temperatures weren’t as brutal as previous nights, both Djokovic and Aussie battler John Millman were dripping wet out on Arthur Ashe Stadium.

In the second set Millman, who brilliantly beat Roger Federer in the previous round, had to ask the umpire permission to leave the court because he was worried about the sweat making the ground too dangerous.

There was no way, however, that the 13-time Slam winning Serbian was about to slip up.

Djokovic eventually ran out a deserved 6-3 6-4 6-4 winner and will now play Japan’s Kei Nishikori on Friday for a place in the final.

He missed last year’s tournament through injury but this will be his 11th straight Flushing Meadows semi-final.

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Djokovic then took a swipe at US Open bosses.

He snapped: "This is the sweatiest tournament I have ever been at. I have never sweated as much as I have here. It’s incredible. I need ten shirts for a match. The tournament needs to address it - the stadium feels like a sauna.

They beat Americans Christina McHale and Christian Harrison in a third set tie-break and the Scot is just one win from landing his sixth Slam title.

Mattek-Sands, who has recovered from a horror knee injury sustained at Wimbledon in 2017, said: “Jamie kept me calm, cool and collected.

“I tend to get a little bit hyper and energetic. He’s the yin to my yang. He came up with some awesome volleys, so we’re pumped to get that win.”

Japanese star Naomi Osaka is through to the women’s last four - making it a stellar day for tennis in Asia following Nishikori’s fine performance in his gutsy five-set win over Marin Cilic.

Serena Williams, who beat Karolina Pliskova, faces Anastasija Sevastova today in her semi-final and is just two wins away from winning her 24th Grand Slam and levelling Margaret Court’s all-time record.